Non-destructive inspection (NDI) of structures involves examining a structure without harming the structure or requiring significant disassembly. Non-destructive inspection is typically preferred to avoid the time and costs associated with the removal of a part for inspection and to avoid the potential for causing damages when inspection is needed. Non-destructive inspection is used in the aircraft industry to inspect aircraft structures such as composite structures and bonded panels. Inspections may identify defects such as cracks, discontinuities, disbonds between layers, voids, and areas having undesirable porosity. Preventive inspections may be performed during manufacturing and at any time during the service life of an aircraft structure to validate the integrity and fitness of the structure. Inspections may also be prompted by incidents such as collisions and ballistic impacts that are suspected or known to cause damages.
Various types of ultrasonic methods are used to perform non-destructive inspections. For example, a structure may be inspected by a pulse-echo (PE) method wherein a sensor device sends ultrasonic pulses into a structure and receives echo pulses that reveal the condition of the structure. Data acquired by such a sensor device can be processed and presented to an operator. B-scan images can be produced that reveal depth characteristics of an inspected structure. C-scan images can be produced to reveal a mapping of the inspected structure. These images can reveal features that are not easily perceived or characterized by simple visual inspection of the exterior of a structure. Collecting data for B-scan and C-scan images typically entails physically moving a sensor along a portion of a structure in order to collect data across an area of the inspected structure. Thus, collecting data for such images can be time-consuming and physically challenging activities.
An ultrasonic sensor can be moved over a structure manually by a trained technician. For example, the technician may move a sensor side-to-side along one axis in order to collect data for a B-scan image. The technician may move a sensor along two axes in order to collect data for a two-dimensional mapping such as a C-scan image. The quality of the inspection depends significantly upon the performance of the technician. Thus, manual scanning of structures is time-consuming, labor-intensive, and prone to human error.
Automated inspection systems have been developed at least in part to relieve technicians of the challenges of manually moving inspection sensors across a structure. For example, the Automated Ultrasonic Scanning System (AUSS®) system includes a complex mechanical scanning system that can perform through-transmission ultrasonic (TTU) inspections. The AUSS system has robotically controlled probe arms that can be moved along opposing surfaces of a structure. Typically, one probe arm moves an ultrasonic transmitter along one surface of a structure, and another probe arm moves an ultrasonic receiver along the opposite surface in concert such that the transmitter and receiver are kept in alignment. To maintain the alignment, a complex positioning system provides motion control in numerous axes. Thus, typical AUSS systems include robotic components and control systems that can be expensive to obtain and challenging to set up and use for any given inspection. Furthermore, access to both sides of a structure must be available if the structure is to be inspected by TTU methods.
Two-dimensional arrays of ultrasonic pulse-echo sensors have been developed and employed in medical imaging procedures. Such arrays provide numerous sensors regularly distributed across an area and each sensor can collect location-specific data. Thus, a mapping of a portion of the interior anatomy of a medical patient can be obtained without movement of the sensors.
It would be advantageous to provide non-destructive inspection methods for inspecting a structure by disposition of a sensor array across an area of the structure without further movement of the array. It would be advantageous to provide one-sided inspection methods for detecting and characterizing damage beneath exterior surfaces of contoured structures such as aircraft wings and fuselages, bridge columns, and boat hulls. Furthermore, it would be advantageous to provide methods of applying pre-determined criteria in selecting a mode of returning a structure to usage, for example returning an aircraft structure to flying service.